FIG. 1 is a cross-sectional diagram of a prior art pressure sensor device 100. A metal port 102 has an exterior surface 104, threaded 106 to allow the port 102 to be attached to an orifice carrying a pressurized fluid.
A cavity 108 extends upwardly through the port 102 and is “closed” at a top side 116 by a thinned area or portion 110, above which is a conventional MEMS silicon pressure sensing element 112. Prior art MEMS silicon pressure sensing elements are disclosed in U.S. Pat. No. 6,427,539 entitled, “Strain gauge,” U.S. Pat. No. 8,302,483 entitled, “Robust design of high pressure sensor device,” and U.S. Pat. No. 8,171,800 entitled, “Differential pressure sensor using dual backside absolute pressure sensing,” to name a few, the contents of which are incorporated herein by reference.
The thinned portion 110 has a thickness that is about 0.3 mm to about 0.4 mm. It acts as a diaphragm, deflecting upwardly and downwardly responsive to changes in the pressure of a fluid in the cavity 108.
The thinned portion 110 is generally planar. The cavity 108 below the thinned portion 110 is preferably a tube or tubular and encircles or encloses a perimeter 114, which is provided with a radius where the wall defining the tubular cavity 108 meets the thinned portion 110 to reduce stress concentrations.
The MEMS silicon pressure sensing element 112 is essentially centered above the perimeter 114. The MEMS silicon pressure sensing element 112 is attached to the top 116 of the port 102 by a glass frit 118. The glass frit 118 bonds or attaches the MEMS pressure sensing element 112 to the top surface 116 such that deflection of the thinned area 110 causes the MEMS silicon pressure sensing element to change its size and shape. When the size and shape of the piezoresistors embedded in the sensing element 112 changes, their resistance values also change, causing an output voltage from the sensing element 112 to change proportionately to the deflection of the thinned area 110.
The port 102 is surrounded by a plastic spacer 120, on top of which is a conventional printed circuit board (PCB) 122. The PCB 122 is attached to the spacer 120 by an adhesive 124. The PCB 122 supports an application-specific integrated circuit (ASIC) 130.
On the left side of FIG. 1, the spacer 120 and PCB 122 support a chip capacitor 126, which is attached to the PCB 122 by an electrically conductive adhesive 127. Thin bond wires 128 extend between the MEMS pressure sensing element 112 and the ASIC 130. Similarly, a second set of bond wires 132 extend between the ASIC 130 and bond pads 134 on the top surface of the PCB 122.
The metal from which the port 102 is made and the material from which the PCB 122 is made, have significantly different coefficients of thermal expansion. (CTE). The coefficients of thermal expansion of the glass frit 118 and MEMS pressure sensing element 112 are also significantly different from the coefficient of thermal expansion for the metal port 102. The mismatches between the CTEs create thermally-induced stresses and voltage noise. In addition to thermally-induced stresses and voltage noise, a threaded connection is difficult to seal hermetically. Reducing or eliminating the mismatch between coefficients of thermal expansions and simplifying the packaging would be an improvement over the prior art.